This invention relates to the control of an electric motor having an armature and field winding powered from a source of direct current.
Various control systems have been devised for direct-current motors, such as are used for propulsion purposes in lift trucks or other vehicles, to vary the current to the motor. Typically, these systems utilize a silicon controlled rectifier (SCR) control circuit which regulates the voltage applied to the motor. When it is desired to apply full power to the motor the SCR control circuit is bypassed so that the motor is connected directly across the battery and full battery potential is applied to the motor.
When operating in the bypass mode with constant full battery potential applied to the motor and with full armature current going through the field winding, a series-connected direct-current motor will have a maximum speed dependent upon the torque required to propel the vehicle in the direction the motor is propelling the vehicle. It is desirable under low torque requirements to provide field shunting whereby a portion of the armature current is shunted around the field winding so that the field strength is weakened. Since the motor speed is inversely proportional to the field strength, field shunting will provide additional speed over full-field speed.
The field shunting control should also be sensitive to the torque requirement so that if the motor is operating in a weakened field mode and the torque requirements increase (as when the vehicle goes up a slope) full field will again be applied so that the motor will deliver maximum torque.